Shoulder Kinematics in Yoga, Part I

The shoulder is the most mobile joint in the body. It is a combination of several articulations: the glenohumeral, scapulothoracic, sternoclavicular, and acromioclavicular joints (click here to see these). Each of these contributes to shoulder and arm movement via a process known as joint “coupling.” For example, when you raise the arms over the head in Urdhva Hastasana, the clavicle pivots on the sternum (breastbone), the scapula rotates on the chest wall, and the head of the humerus moves within the shoulder joint. All of these work together simultaneously in what is known as scapulohumeral rhythm. In general, 60 degrees of this action comes from movement of the shoulder blade on the chest wall (scapulothoracic motion) and 120 degrees from the ball and socket of the shoulder (glenohumeral motion). During this process, the clavicle rotates on its long axis approximately 25 degrees and the humerus externally rotates about 45 degrees. Scapulohumeral rhythm is illustrated in the video below.

So we can see that the shoulder is a complex structure, with many factors contributing to movement, or potentially restricting it. It’s easier to understand a complex structure by breaking it down into its component parts and then reconstructing them into the whole. This gives a new understanding of their function. For the shoulder, we’ll begin by focusing on the skeletal anatomy of the glenohumeral joint.